The present invention relates to an improved sensor for producing a signal indicating the moisture content of material into which the sensor is inserted.
Many applications exist in which it is necessary to accurately ascertain the moisture content of material. For example, knowing the exact moisture content of soil is important in agricultural applications, in waste treatment applications where soil is used for purifying water, for erosion studies, for monitoring earth dams, and for various military applications. Accurately determining the moisture content of grain is another important application. Maintaining accurate records as to the amount of moisture in trees is not only important in preventing forest fires, but also in monitoring the growth of those trees. Many other industrial applications such as curing concrete also require periodic and accurate information as to the moisture content of material.
Typically, moisture sensing devices in the past have included a container into which the material is placed, with plates or the like therein for determining the capacitance of the material placed therein and relating that capacitance to the moisture content. For example, the U.S. Pat. No. 3,209,247 to Mead and the U.S. Pat. No. 4,050,016, to Marsh et al show typical devices of this sort. These devices are, however, inconvenient to use since they require removing a portion of the material to be tested. Further, removing the material, for example, digging a sample of soil, necessarily changes its density so that the measured results are not necessarily the actual moisture content of the soil before its removal.
Another inaccuracy arises in many of these devices because they measure only the capacitance of the soil or measure only the resistance. U.S. Pat. No. 3,803,570 to Barlow et al describes a capacitance measuring device. None of these capacitance devices however have effectively combined high accuracy with ease of use. The U.S. Pat. No. 2,870,404 to Oxley describes a resistive measuring device in which a plurality of spikes are inserted into the ground. In fact, both the resistance and capacitance of the soil vary with moisture and vary independently of each other depending upon soil condition. The relation of resistance to moisture particularly is non-linear and very difficult to predict for any given composition. Devices which ignore variation of resistance with capacitance necessarily produce an inaccurate indication of moisture content.
My U.S. Pat. No. 4,288,742 issued Sept. 8, 1981 discloses a unique, simple, and effective moisture sensor which can be inserted easily into material to be measured, usually without damage to that material, and which takes into account both resistance and capacitance to produce an accurate indication of moisture content. The sensor includes a probe having at least a single, and preferably a plurality of spines extending outward from a base so that the spines can be inserted into the material. The spines are sufficient in number to appear as a ground plane forming an effective coaxial capacitor. Inaccuracies resulting from fringing fields are eliminated while the device remains easily insertable.
The impedance produced by the material surrounding the spines forms part of an RC bridge, preferably a Wien or other bridge, which also includes a separate resistor and capacitor. Thus, the impedance of the material, both its capacitance and resistance, are measured to produce signals indicating that impedance. By determining the ratio of the voltages across the RC circuit forming part of the bridge and the RC circuit of the material impedance and determining the resonant frequency, both the resistance and capacitance of the soil can be determined and related to the dielectric constant of the material. From that dielectric constant the soil moisture content can be easily determined according to well known relations.
The co-axial geometry accurately defines the active volume by minimizing fringe volumes. With sensors of the type which use plates, the fringe capacitances introduce errors since those capacitances vary with the dielectric constant. The co-axial geometry has no such fringe capacitance, except at the ends. A first ring of spines extend outwardly from a base in parallel with a second ring of spines extending outwardly from the base, also in parallel, and within the first ring, separated and insulated electrically therefrom. The two rings thus form an effective coaxial capacitor which can be inserted into the material to be sensed.
While the sensor described in the above-mentioned patent is quite satisfactory and superior to other techniques generally reviewed above, the use of individual spines to form the coaxial capacitor has at least two disadvantages. First, the configuration is somewhat difficult to mechanically construct. The individual spines must be formed and accurately attached in a permanent way to a metal or other ring or the like. Second, there is some possibility of dislocation of the spines as they are pushed into the ground and perhaps encounter some object or, for some other reason, are slightly displaced. This displacement also produces some inaccuracies in the final output although such inaccuracies are not necessarily unacceptable.
The present invention relates to an improved moisture sensor in which these above-noted difficulties are eliminated by the use of a member extending outwardly from a base to define extending surfaces which partially bound a volume containing the media when inserted. An opening is provided between the volume and the media outside the sensor to permit movement of moisture therethrough and also make insertion easier.
In one embodiment two slotted cylindrical tubes are mounted coaxially and replace the spines described in the above-mentioned application. The cylinders are sharpened on the end which pushes into the ground. A simple insulating plug can be used to mount and electrically separate the two sensors. Since the cylinders have greater rigidity than the previously used spines, the small problem of displacement does not occur as readily. Forming slots in the cylinders is a much easier machining process than forming and mounting the spines as in the previous approach.
In a second embodiment the volume is partially bounded by a cross-shaped member having flat surfaces defining the volume in cross-section as a square center with a rectangular leg extending from each side thereof. Each leg is open at the peripheral edge. The member and the volume in the legs tapers in the longitudinal direction so that at the insertion end the volume is made up only of the center section. A plurality of parallel plate capacitors are thereby formed by the parallel facing surfaces which are driven at the same potential. The four legs provide not only a controlled volume but good mechanical rigidity. The outer part of each leg can be insulated from the rest of the member if desired to serve as a guard ring.
In a third embodiment two cross-shaped members are joined together at the peripheral edge of one leg of each. This gives better definition of the electrical volume since more of the volume is remote from openings and therefore less susceptable to fringe effects.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.